Time domain reflectometry has been previously known effective in methods for determining the level of a liquid, such as in a tank. According to such time domain reflectometry methods, electrical pulses are conveyed along a transmission line to an electrically conductive probe extending over the range of liquid levels being detected. The stimulating electrical pulses produced in the time domain reflectometry system are partially reflected at the vapor-liquid interface due to a change in the electrical impedance. The impedance change is associated with the differences in the dielectric strength between the liquid and the overlying gas or vapor. The electrical permittivity is the technical term indicating the dielectric properties of the fluids involved.
The electrical pulses produced by a time domain reflectometry system are affected by the dielectric constant of the surrounding media in which the signal is traveling. The dielectric constant (permittivity) of the adjacent media directly affects the propagation velocity of an electromagnetic wave as it travels along the transmission line and along any attached probe or sensor. In time domain reflectometry systems, a fast rise time electromagnetic pulse is propagated along a transmission line having a known length while measuring the time of arrival and the time of reflections from electrical discontinuities in the transmission line at two known, spaced points. One known, spaced point is located where a coaxial connecting cable of the transmission line is attached to the transmission line probe. The other known, spaced point is located at the distal end of the transmission line probe. Since these locations are both known, one can calculate the propagation velocity of the electromagnetic wave and, as a result, calculate the apparent dielectric constant of the material undergoing tests and through which the transmission line probe extends. Similarly, changes in the dielectric constant which relate to changes in the media adjacent the probe can also be determined. For example, the apparent dielectric constant may provide a direct indication of the presence of water versus the presence of water vapor or air.
U.S. Pat. No. 4,786,857 to Charles L. Mohr, et al., entitled "Methods and Apparatus for Time Domain Reflectometry Determination of Relative Proportion, Fluid Inventory and Turbulence", disclosed apparatus and methods for using time domain reflectometry to determine the relative proportions of intermixed constituents in a fluid system. Such apparatus and methods can be used to determine the relative proportions of liquid and vapor even when the liquid and vapor are intermixed either homogeneously or non-homogeneously. Measurement capabilities such as these are particularly valuable to the process industries and nuclear energy production. The systems can be used to monitor nuclear reactor coolant systems, in which the total inventory of system coolant, including intermixed water and steam, must be determined under a variety of conditions, including even accident conditions. Methods are also described for obtaining indications of turbulence in fluid mixtures by measuring variations in fluid properties over time.
The above-mentioned Mohr patent disclosed a probe including an inner centrally located electrode mounted within a cylindrical outer electrode. The cylindrical outer electrode was provided with slots to allow fluid to pass into the annular volume between the inner and the outer electrodes. The probe was immersed in the mixed-constituent system. The average dielectric constant or permittivity experienced by the electrical pulse transiting the probe was determined using time domain reflectometry. The measured permittivity was then correlated with known characteristic data of the constituents being measured to determine their relative proportions.
U.S. Pat. No. 5,554,936, also to Charles L. Mohr, et al., entitled "Mixed Fluid Time Domain Reflectometry Sensors", disclosed apparatus in the form of improved probe sensors which could be used for a greater variety of applications and still provide measurements. More particularly, there was a need to provide a probe that was more effective when used in some applications, particularly in applications where solutions rich in minerals, such as from earth wells, were not capable of measurement. Accordingly, the improved probe sensor was capable of service under a variety of conditions with accuracy and reliability.
The probe shown in U.S. Pat. No. 5,554,936 has been found less than satisfactory when used in some situations. One situation is when the probe is required to be placed directly within the flow path of a fluid flow channel. Placement of the probe sensor directly across a flow channel subjects the probe to pressures from fluid flow, increases the risk of damage from the flowing fluid and materials present within such flow. Placement across a flow channel also requires that the probe sensor be removed during cleaning operations of the fluid flow channel in order to prevent damage to such probe. The current invention addresses the need for improved time domain reflectometry probes which are capable of service under a greater variety of conditions in fluid flow channels, with accuracy and reliability.